Method and Apparatus for Detecting Possible Arterial Constriction by Examining an Eye

ABSTRACT

A tonometer capable of measuring systolic or ocular pulse pressure (OPP) can be used to examine eyes of a patient to measure whether OPP or ocular blood flow (OBF) is sub-normal to one or both eyes. This can serve as an indication of a possible constriction of blood flow in arteries leading not only to the eyes, but to the brain and other regions above the neck. Any such sub-normal measure can then indicate that further investigation of blood flow in carotid and other arteries is appropriate.

REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of co-pending application Ser. No.10/178,987, filed 25 Jun. 2002, entitled “Method and Apparatus forExamining an Eye”. The aforementioned application is incorporated hereinby reference.

TECHNICAL FIELD

Eye examining instruments and methods.

BACKGROUND

Our previous U.S. Pat. No. 5,070,875, entitled “Applanation TonometerUsing Light Reflection To Determine Applanation Area Size”, and U.S.Pat. No. 6,179,779, entitled “Replaceable Prism System For ApplanationTonometer” and our pending application Ser. No. 09/756,316, entitled“Method of Operating Tonometer”, and Ser. No. 09/811,709, entitled“Replaceable Prism For Applanation Tonometer” suggest tonometers,tonometer operating methods, and tonometer prisms for measuring intraocular pressure (IOP) of an eye. Our type of applanation tonometer hasan actuator that presses a prism with a variable and determinable forceagainst a cornea of an eye being examined while a source directs lightto reflect from an applanation surface of the prism to a detectorproducing a detected light signal received by a microprocessor.

Our experiments and experiences with working prototypes improving uponthe disclosures of our issued patents and pending applications have ledto several related discoveries. We have found that by changing andadding to the eye examining procedures that are possible withinstruments such as ours we can obtain considerably more diagnosticinformation than has previously been clinically available. Thesediscoveries involve not only eye examining methods but also structuringand programming a tonometer to perform such methods to obtain newmeasurements and new information of value to a clinician concerning theheath and functioning of an eye being examined. Such improvements arethe subject of this application.

SUMMARY

The tonometers that are commonly used clinically have operated onlyduring a diastolic phase and have measured only a diastolic intra ocularpressure (IOP). In contrast to this, we have discovered that ourinstrument can produce a useable signal during a systolic pulseoccurring in an eye being examined. Upon exploring this, we found that asystolic phase signal from our instrument can be used to determine anocular pulse pressure or a systolic IOP. This constitutes valuableadditional information not obtainable with previous clinical tonometers.It provides a measure not only of diastolic IOP, but also of systolicIOP, and enables an average, weighted average, or mean IOP determinationthat more accurately represents the true or complete IOP experienced bythe eye being examined.

Other experiments with IOP signals attainable from our instrument haveled to eye examining methods differing from our previous suggestions. Wehave found, for example, that IOP can be determined from a slope of asignal obtained as prism pressing force is changed during a timeinterval. This has eliminated any need to applanate an eye to apredetermined applanation area.

We have also found that a prism pressing force variation range for IOPexamining purposes can begin with a preliminary value and change fromthat value through a predetermined signal change range, rather thanproceeding from a reference applanation area to a measurementapplanation area. This method eliminates variations in corneal thicknessand curvature of different eyes, since these variations areautomatically compensated for by the preliminary value from which thepredetermined signal excursion range proceeds.

Experience with systolic pulse signals produced by our instrument hasled to discovery of other measurements available from examining an eye.We found that we can determine ocular blood flow derived from thedeparture of the detected light signal from the diastolic IOP during thesystolic pulses. Moreover, we have found that we can determine atonography measure from the way the detected light signal changes from asystolic pulse back to the diastolic phase. We can also determinetonography by measuring a preceding IOP; then pressing the prism againstthe eye with a predetermined force sufficient to raise the IOP for apredetermined interval; followed closely by determining a subsequentIOP. From this we can derive the tonography measure from the differencesbetween the preceding and the subsequent IOP determinations. An ocularblood flow measurement, and a tonographic measurement of theeffectiveness of an eye's trabecular meshwork add significant andpreviously inaccessible diagnostic information of value to a clinician.

The ability of our instrument to determine ocular pulse pressure (OPP)and ocular blood flow (OBF) can also be exploited to give evidence ofblood flow in the carotid and other arteries supplying organs above theneck. If tests with our tonometer indicate sub-normal blood flow in oneor both of the eyes, this can indicate possible constriction of bloodflow in arteries leading not only to the eyes, but also to the brain andother organs of importance. Since examination of a pair of eyes with ourinstrument is quick and convenient in determining OPP and OBF, ourinstrument can serve as an advantageous preliminary test to determinewhether, for example, a carotid artery ultrasound examination should beundertaken. Conversely, examinations with our instrument indicating anormal OPP or OBF for each eye can be considered evidence that anultrasound evaluation of arterial blood flow may not be necessary.

Finally, to ensure that the additional information produced by our eyeexamining method and instrument is readily available to clinicians, wehave made our instrument fast acting, compact, convenient, and objectivein its operation. Besides producing much new information, our instrumentautomatically rejects false readings, and automatically requiresconcentric contact with a cornea at a proper orientation to attain anaccurate reading. The microprocessor in our instrument can preferablystore, send, and receive information to perform all the required tasksand operations and to co-operate with computers and other informationprocessing equipment.

DRAWINGS

FIG. 1 is a schematic view of a preferred embodiment of our improvedtonometer, which is suitable for practicing our inventive eyeexaminations.

FIG. 2 is a schematic diagram of a portion of the tonometer of FIG. 1involving a prism applanating an eye, a light source, a detector, amicroprocessor, and an output.

FIGS. 3-7 are schematic graphs of detector signals produced pursuant toour inventive eye examinations.

DETAILED DESCRIPTION

Our eye examining method requires an instrument that can produce asignal representing intra ocular pressure (IOP) of an eye beingexamined. Such instruments are normally called tonometers, but ourinstrument and the ways it can be used produces information going beyondwhat can be expected of previous tonometers. Several variations oftonometers suitable for our purposes are described in our previouspatents and applications. A presently preferred embodiment of such atonometer 10 is schematically represented in FIGS. 1 and 2. Tonometer 10preferably includes an actuator 15 pressing a prism 30 with a variableand determinable force against a cornea of an eye while a light source20 directs light to reflect from an applanation surface 31 of prism 30to a detector 25. In response to received light, detector 25 produces adetected light signal that is electrical in form and is sent to andanalyzed by microprocessor 50 which in turn controls actuator 15 forvarying the force of prism 30 pressing against the eye 40 and forvarying the energization of light source 20. Microprocessor 50 alsopreferably drives a display 51 and preferably communicates with outputand input devices 52.

The essential components of tonometer 10 include microprocessor 50,prism 30, some form of actuator 15, a light or radiation source 20, anda transducer or detector 25 receiving light or radiation reflected fromapplanation surface 31 and sending a corresponding electric signal tomicroprocessor 50. The precise working relationships among theseessential components can be varied considerably, however, and theschematic illustrations of FIGS. 1 and 2 show only a presently preferredembodiment.

Prism 30 is preferably replaceable and disposable so that it can beremoved from prism holder 32 after each examination and replaced with afresh prism. This ensures that infectious agents, including thepossibility of prions, are not transmitted from one pair of eyes toanother.

Prism holder 32 is preferably mounted on arm 12, which is arranged torotate or turn slightly around pivot 13. Since only a few millimeters ofmovement back and forth of prism 30 is required, as indicated by thedouble headed arrow, the rotational turning of arm 12 is slight.

A counter balance 14 arranged on arm 11 is arranged with a suitablemoment arm relative to pivot 13 to return prism holder arm 12 to a baseposition. Proper arrangement and balancing of arms 11 and 12 aroundpivot 13 can eliminate the need for any return spring, and can enableinstrument 10 to operate in different orientations.

Actuator 15 can be any of a variety of motors and other preferablyelectromagnetic prime movers. The preferred actuator schematicallyillustrated in FIG. 1 includes a coil 16 that is moveable relative to apermanent magnet 17, depending on the power supplied to coil 16. Therelatively lighter coil 16 is preferably fixed to arm 12, and therelatively heavier permanent magnet 17 is preferably fixed to a body ofinstrument 10, because this reduces the rotating mass. It might also bepossible to arrange permanent magnet 17 or coil 16 in the position ofcounter balance 14 to further reduce the rotational mass. Many otherpossibilities exist but for sake of conciseness and simplicity have notbeen illustrated.

Internal wiring within instrument 10 preferably connects a power supply(not shown) with microprocessor 50, which powers actuator 15, lightsource 20, and light detector 25. Microprocessor 50 preferably drivesdisplay 51 to display information directly to an instrument operator,and microprocessor 50 preferably has connections enabling it to receiveand output information to and from other devices such as computerkeyboards and number pads 52.

The light emitted by source 20 is preferably in a visible red region ofthe electromagnetic spectrum, but other colors of visible light are alsopossible, as is radiation energy at infra red or microwave frequencies.For simplicity, all of these possible different radiation frequenciesare characterized as “light” within this application. As is well known,use of electromagnetic radiation at different frequencies can requireangular adjustments so that light within prism 30 that is internallyincident on applanation surface 31 is internally reflected to detector25 except for a portion of the light that transmits into eye 40 throughan applanation region of contact with prism surface 31. Of course, anyradiation entering eye 40 must not cause injury.

Prism 30 is preferably molded of resin material to have a low enoughmanufacturing cost to be affordably disposable. Much more information onpreferred characteristics of prism 30 is available in our U.S. Pat. No.6,179,779 and our pending application Ser. No. 09/811,709. As we havepreviously suggested, microprocessor 50 is preferably programmed torequire that prism 30 be replaced after being used for examining a pairof eyes. This is intended to prevent the tonometer prism fromtransporting infectious agents from the eyes of one person to the eyesof another.

Experiments and clinical experience with a working prototype of atonometer instrument such as shown in FIGS. 1 and 2 has led to a simplerway of measuring a conventional diastolic intra ocular pressure (IOP).Our previous patents and applications recommended determining IOP fromprism pressing force required to change between a reference applanationarea (with a corresponding reference signal) and a measurementapplanation area (with a corresponding measurement signal). We have nowfound that it is not necessary to use predetermined reference andmeasurement areas and corresponding signals to determine IOP.

Our new method preferably begins with a preliminary value of a detectedlight signal that the tonometer microprocessor is programmed torecognize. The preliminary value is preferably based upon preliminarycontact of the prism applanation surface with a cornea at either barecontact pressure arising from surface tension of tears in the eye, orfrom a preferably very light predetermined prism pressing force.

The tonometer can be programmed to recognize that the preliminary valuehas occurred by noting the sudden reduction in the detected signal thatoccurs when the prism contacts the eye. Before this happens, all thelight incident on prism applanation surface 31 is internally reflected,but when surface 31 contacts a cornea, a portion of the incident lightis transmitted into the eye in the area of contact so that the detectedlight signal diminishes noticeably. The preliminary value signal canvary from eye to eye, as explained below, but can also serve as astarting point for an IOP determination.

Then instead of proceeding from the preliminary value to a measurementvalue, we proceed from the preliminary value through a range of valuesextending from the preliminary value. Any value at the end of the rangeextending from the preliminary value is not a fixed value, but is basedonly on distance from the preliminary value. To accomplish this, weprogram the microprocessor to operate actuator 15 to press prismapplanation surface 31 against the cornea with increasing force appliedduring a time interval to change the detected light signal from thepreliminary value through a range of values extending from thepreliminary value. In doing this, microprocessor 50 preferably energizesactuator 15 to apply increasing prism force in predetermined incrementsthat are applied in predetermined brief time intervals so that thedetected light signal 26 varies in a step wise, sloped configuration asgraphically illustrated in FIG. 3. Prism force changes and timeintervals can also be continuous or analog, rather than being brokeninto the preferred increments. Also, prism force changes with respect totime are preferably linear, rather than nonlinear, to simplify slopedeterminations.

Signal 26 then provides several ways of determining IOP. One way is toproceed with increasing prism pressure force for a predetermined numberof increments or for a predetermined time interval resulting in apredetermined increase in prism pressing force above the preliminaryvalue. We then program microprocessor 50 to determine IOP from thesignal change reached at the end value of the predetermined range ofprism force change values. By a similar method, the force change valuescan be continued until the detected light signal 26 reaches apredetermined departure from the preliminary value, with microprocessor50 determining IOP from the total pressure force required to reach theend signal value. Such an end signal value differs from the previouslysuggested measurement signal value by being related to the preliminarysignal value 27 rather than being an absolute or independent value.

Another way that microprocessor 50 can determine IOP from detected lightsignal 26 is by measuring or detecting the slope as signal 26 changesover a time or force interval. We have found that signal slope alone isenough for microprocessor 50 to make an accurate determination of IOP.We have also found that detected signal slope tends to roll off athigher prism forces, so we prefer using a linear mid-region or lowerforce region of detected signal 26 for an IOP determination.

From an ophthalmologically known relation between IOP and force used inapplanating a corneal area we have been able to apply linear,logarithmic, and logistic regression analyses to calculate IOP from thesignals produced by instrument 10. In these analyses we have used theslope of the detected signal as changes in prism force change cornealapplanation area and cause a corresponding change in signal value.Especially logistic regression analysis, which allows us to considerseveral variables at a time, has been useful in makingforce-to-signal-to-IOP calculations. We have also corroborated theseresults by manometric comparisons and Goldman tonometer readings.

Microprocessor 50 is preferably not programmed to make sophisticatedmathematical calculations itself. We prefer instead that signal analysisbe done separate from microprocessor 50, which is then programmed orloaded with a look up table from which it can determine IOP based onsignal values. In doing this, microprocessor 50 can be programmed toaverage an IOP determination made by different methods.

These IOP determining methods can also be combined or used inconjunction so that one determination corroborates another. Moreover,elapsed time required to reach an end value can serve as anothercorroborator of an IOP determination. All the IOP determining methodscan be combined in a single instrument that determines IOP according toeach method and corroborates by comparing elapsed time, force change andelectrical signal change. Any differences in the IOP determinations canbe averaged out, unless differences are unusually large, in which casethe microprocessor can be programmed to repeat the measurement. In asimilar way, if elapsed time casts doubt on the accuracy of an IOPdetermination, the examination can be repeated.

These methods of IOP determination, besides being simple, accurate andfast, have another important advantage. By monitoring signal changerelative to a predetermined value 27 that is not fixed but is related toeach eye being examined, these IOP determining methods automaticallytake into account variations in corneal curvature and thickness. Ourprevious IOP determining suggestions envisioned separate measurements ofcorneal curvature and corneal thickness and input of such measurementsto adjust IOP determinations. This is no longer necessary with ourpresent IOP determining methods, because the preliminary signal value 27is not a fixed value but is allowed to vary with each eye beingexamined.

This variation is schematically illustrated in FIG. 3 by the differencebetween preliminary value signals 27 and 27 a, and the correspondingdifference between detected signals 26 and 26 a. More specifically,preliminary signal value 27 can be seen as a typical signal produced bycontact of the tonometer prism with the cornea of a normal eye having anormally curved or arched cornea and a normal cornea thickness.Preliminary value signal 27 a then indicates a preliminary contactsignal from a cornea that is flatter or less arched than normal or acornea that is thinner than normal. The value of preliminary signal 27 abeing less than the value of preliminary signal 27 indicates thatpreliminary prism contact with the thinner or flatter cornea hasapplanated a larger area. It would also be possible, but is omitted forthe sake of simplicity, to show another preliminary value signal largerthan signal 27, indicating preliminary contact with a cornea that ismore arched than normal or is thicker than normal, resulting inapplanation of a smaller area.

The ophthalmological literature indicates that every 50 micrometervariation in corneal thickness produces a 1.5 mm Hg variation inmeasured IOP. This variation will automatically appear in preliminaryvalue signal 27 to adjust the starting point for a predetermined rangeof values. Similar indications are available in the ophthalmologicalliterature for the effects of differences in arching or curvature of thecornea. These two translate into differences in measured IOP, which areautomatically compensated for by preliminary signal value 27.

The variations in preliminary signal 27 due to corneal characteristicssuch as curvature and thickness do not matter in our present method ofdetermining IOP, because change in detected light signal 26 or 26 a,proceeds through a range extending from whatever preliminary valueoccurs. This automatically eliminates corneal curvature and thickness aspossible variables in an IOP determination. The result is a moreaccurate IOP determination that does not have to be adjusted for cornealthickness or curvature and does not require any separate measurements ofor adjustments for corneal thickness and curvature. This advantage canbe especially important in examining eyes whose corneas have beenmodified for vision correcting purposes. The fact that such corneas canrespond as thinner than normal does not prevent our tonometer fromaccurately measuring IOP.

We have also found that our tonometer instrument detects systolic pulsesduring eye examinations. Goldman tonometers, which are ubiquitous inopthalmological clinics, cannot make an IOP reading during a systolicpulse. Our instrument, in contrast, produces a detected light signalduring both a diastolic phase and a systolic phase. This has led toseveral new ways of determining IOP, one of which is schematicallygraphed in FIG. 4.

The process illustrated in FIG. 4 occurs after making a determination ofdiastolic IOP. For doing this, microprocessor 50 is preferablyprogrammable to allow an operator to secure a reading only of diastolicIOP. Ocular pulse signals occurring during a systolic phase are thenignored by microprocessor 50 in making an IOP determination.Microprocessor 50 can also be programmed to proceed in several ways todetermine a diastolic IOP while also taking into account the detectedsignal departure from a diastolic phase 26 during a systolic pulse.

For the method illustrated in FIG. 4, microprocessor 50 is programmed tooperate actuator 15 to hold prism 30 against a cornea with apredetermined force for a long enough duration for two systolic pulses28 to occur. These are illustrated in FIG. 4 as saw-tooth shapedincreases in the otherwise flat diastolic signal 26. The height ofsystolic pulse signals 28 above diastolic signal 26 indicates ocularpulse pressure (OPP), and this is preferably used as an ingredient in anIOP determination. The fact that systolic pulse signals 28 are larger orhigher than the diastolic phase signal 26 is because a systolic pulseslightly hardens the eye being examined, which reduces the areaapplanated by the tonometer prism 30 and increases the detected lightsignal.

The eye is subject not only to the IOP occurring in a diastolic phase,but also to the increased IOP that occurs during a systolic pulse when abolus of blood flows into the eye. An IOP determination based only on adiastolic phase measurement therefore does not indicate the truepressure experienced by the eye over time. For example, a diastolicpressure might be 20 mm of mercury, while systolic pulse pressures reach26 mm of mercury. A true IOP reading for all the pressure experienced bythe eye over time should then include the systolic pulse pressures as afactor increasing the IOP over the diastolic phase pressure.

Broken line 29, as illustrated in FIG. 4, indicates an average, weightedaverage, or mean IOP determined not only from diastolic phase signal 26,but from the increased height of ocular pulse signals 28, and from apulse rate. Microprocessor 50 is preferably capable of measuring timeintervals and of calculating a pulse rate from the time elapsed betweena pair of OPP signals 28. Microprocessor 50 then preferably determines amean or average IOP having a value higher than a diastolic phasemeasurement, based on the height and frequency of systolic pulsepressures. The systolic or OPP is calculated in a manner similar to thecalculation of the diastolic IOP. The OPP can also be calculated as apercentage of the deviation from the diastolic phase IOP. Variousregression analyses can be applied to these calculations, which can becorroborated manometrically.

Microprocessor 50 can also be programmed to determine only a systolicIOP, which can be done by ignoring diastolic signal phase 26. For mostpurposes, though, an average, weighted average, or mean IOPdetermination based on both diastolic and systolic phase signals ispreferred.

The ability of our instrument 10 to produce usable systolic pulsesignals 28 can also be used to determine ocular blood flow (OBF). Thisis preferably determined from the height of systolic pulse signals 28above diastolic base line 26. The ophthalmological literature includessuggestions for calculating ocular blood flow from ocular pulse pressureby using a Friedenwald equation or one of the suggested modifications ofthe Friedenwald equation. Most of these suggestions focus on the heightof signal 28 above a diastolic base line 26, as we prefer. Somesuggestions have also focused on a leading edge of signal 28, and atleast one proposal, which we have not adopted, suggests that the areaunder the signal 28 above the diastolic baseline 26 be considered. Thevarious forms of regression analysis can also be applied to ocular bloodflow calculations, and these can be corroborated by clinical experience.Whatever calculations are used, the results are preferably translatedinto a look up table programmed into microprocessor 50 for use inoutputting OBF information.

Another measurement that becomes possible from the availability of OPPsignals 28 is a tonography measure of the effectiveness of thetrabecular meshwork of the eye being examined. We prefer determiningthis from the downward or trailing slope 24 of the OPP signal 28 as itreturns from a peak pressure back to the diastolic base line 26.Generally, a steeper downward slope 24 indicates an effective trabecularmeshwork that quickly returns from an elevated OPP back to a diastoliclevel. Conversely, a more gradual and extended downward slope 24indicates a trabecular meshwork that is more impaired and recovers moreslowly from a systolic pulse pressure.

Methods of calculating tonography measures from applanation signals arealso available in the ophthalmological literature. These generally agreethat down slope 24 is a key ingredient for tonography calculations. Likethe IOP, OPP, and OBF calculations, tonography measures can be refinedby regression analyses and can be corroborated by clinical experienceand by other measures; they are preferably translated into a look uptable programmed into microprocessor 50.

The availability of a tonography measure, along with an OPP measure andan OBF determination gives a clinician considerably more informationthan has been available from tonometers. This additional information isvaluable for both diagnostic and treatment purposes. Knowing the valueof systolic pulse pressures and average or mean IOP gives informationthat is helpful in setting the aggressiveness of treatments used toreduce IOP. It can also help to determine whether to treat with drugsaimed at improving the effectiveness of the trabecular meshwork orwhether to treat with drugs aimed at slowing down the production ofocular fluid. For example, using tonometer 10 to produce both an IOPdetermination and a tonography determination can affect a treatmentmethod for an eye being examined. If the IOP determination is highenough to warrant treatment and the tonography determination is normal,then a clinician would treat the eye with a drug aimed at reducingproduction of aqueous fluid. On the other hand, if the IOP determinationis high enough to warrant treatment and the tonography determination issubnormal, meaning that the trabecular meshwork of the eye is performingat a rate less than normal in removing aqueous fluid from the eye, thena clinician would treat the eye with a drug aimed at improvingperformance of the trabecular meshwork.

Knowing a measure of OBF can be relevant to these choices because somedrugs can reduce ocular blood flow as a side effect. This must beguarded against so that OBF is not reduced enough to impair the healthof the optic nerve and other eye components. Some drugs used in glaucomatreatment are either known or suspected of reducing OBF as a sideaffect, and having an OBF determination readily available can be used toavoid such drugs in a treatment aimed at reducing IOP.

Some drugs are also known to improve blood circulation generally, andthese can be used if an OBF determination indicates that ocular bloodflow of an eye being examined could advantageously be increased. This isthe case with normal pressure glaucoma that impairs an optic nerve byreducing OBF while an IOP determination remains normal. A clinicianhaving tonometer 10 to make determinations of both IOP and ocular bloodflow can diagnose that deterioration of an optic nerve is caused bysubnormal ocular blood flow, without any increase in IOP. Theappropriate treatment based on the low ocular blood flow determinationprovided by tonometer 10 would then be aimed at improving ocular bloodflow, rather than reducing IOP. Drugs now exist that improve blood flowgenerally, and these can be tried and the results monitored byrechecking the eye for both IOP and OBF. Drugs may also be developedthat will aim especially at increasing OBF to the eyes while minimallyaffecting blood flow elsewhere.

Since OBF determinations have not previously been readily available toclinicians treating eyes for glaucoma, the effect of an OBFdetermination on eye treatment strategies has not been generally known.The ability of instrument 10 to provide such OBF information has manyuses including monitoring an eye under treatment to be sure that thetreated eye is not suffering from reduced ocular blood flow, which wouldcall for a change in drugs being used in treatment. Having an OBFdetermination available from instrument 10 can also be beneficial inverifying that a drug aimed at increasing OBF in an eye experiencingnormal pressure glaucoma has actually improved OBF. Knowing a tonographymeasure of the effectiveness of the eye's trabecular meshwork can alsohelp monitor treatment determinations. For example, this can be used todetermine the effectiveness of drugs intended to improve the working ofthe trabecular meshwork.

OPP and OBF, as measured by instrument 10, can be relevant to the healthof other organs beside the eyes. Blood flow from the heart to the aortaand to the carotid arteries proceeds upward above the neck, not only tothe eyes, but also to the brain and other important organs. If bloodflow in the carotid and other arteries serving the head is constricted,evidence of this may appear in an OPP or OBF measurement readily madeavailable by instrument 10. More specifically, health ramifications of asub-normal OPP or OBF measured in one or both eyes can imply potentialproblems or cardiovascular events such as transient ischemic attack(TIA) or cerebral vascular accident (CVA).

Previous tonometers have not been able to produce OPP and OBF signals,so their use has been limited to diagnosing eye problems. The fast andinexpensive availability of OPP and OBF measurements made possible bytonometer 10 allows OPP or OBF measurements to indicate possiblecardiological problems for the head, generally. Ultrasonic examinationof ocular blood flow in carotid arteries is available, but is acumbersome and expensive test compared with the OPP and OBF informationthat is quickly gained from use of instrument 10. As previouslyexplained for IOP calculations, tonometer 10 is preferably programmedwith look-up tables allowing it to distinguish between and indicate to atonometer user whether OPP and OBF for a measured eye are normal orsubnormal. If instrument 10 detects sub-normal blood flow to one or botheyes, then an ultra sound evaluation and possible treatment areindicated and might be able to forestall stroke risks such as TIA orCVA.

The OPP and OBF determinations available from instrument 10 also allowcomparison of pulse pressure and flow to the two eyes of a singlepatient. An OPP or OBF that is subnormal for one eye and normal foranother can indicate possible constriction of blood flow in an arteryleading to the sub-normal eye. This can indicate that further testingsuch as an ultrasound examination of the appropriate carotid artery, isappropriate.

FIG. 5 illustrates the possibility of producing in a single examinationexcursion a signal that includes a preliminary value 27, a diastolicphase 26, and a systolic phase that includes ocular pulses 28. Such asignal would preferably extend for long enough to include two ocularpulses 28 so that microprocessor 50 can calculate a pulse rate. Ofcourse, it is possible for such a signal to extend for a longer time,but this adds to the time that the prism is pressed against a patient'seye. A typical time needed to complete an examination excursion fordiastolic purposes only is from 0.5 to 1.0 seconds. This would have tobe extended somewhat to insure that two systolic pulses 28 occurred forpulse rate determination.

The signals shown in FIG. 5 can be used to determine an average or meanIOP 29 as previously described, along with ocular pulse signals 28.Although diastolic baseline 26 is sloped because of the signal beinggenerated while prism pressure increases over time, all the informationnecessary for determining IOP in the ways described above is available.An adjustment is needed for microprocessor 50 to take into account thesloping nature of the baseline signal 26 and the corresponding slopingnature of ocular pulse signals 28. Once the slopes are taken intoaccount, all the necessary calculations for diastolic, systolic, andaverage or mean IOP can be calculated, along with ocular blood flow anda tonography measure.

The embodiment of FIG. 6 illustrates the possibility of producing asignal during increasing prism pressure as signal 26 slopes downward,followed by diminishing prism pressure as signal 26 slopes back upward.Such a reversing signal can also extend for long enough to include atleast two ocular pulses 28 from which pulse rate can be determined.Again, after taking into account the slopes caused by prism forcechanges over time, a signal such as illustrated in FIG. 6 can be used tomake all the determinations described above.

The embodiment of FIG. 7 schematically illustrates another way ofdetermining a tonography measure by using tonometer 10. Beginning withpreliminary signal value 27, microprocessor 50 is programmed to increaseprism pressure force in one of the previously described ways to producesignal 36 (from which any systolic pulses have been eliminated) to makea preliminary determination of diastolic IOP. Prism force is thenincreased to produce signal 37, and prism force is sustained at thatlevel for a predetermined interval. The prism force level for producingsignal 37 is sufficient to raise the IOP of the eye to a level above theIOP determination made during the excursion-producing signal 36. Duringthe interval that prism pressure force is elevated, as represented bysignal 37, the eye that is subjected to the extra pressure attempts toaccommodate to return its IOP to normal. The predetermined intervalallowed for this is preferably in the range of a minute or two. Thenprism pressure force is released, as represented by signal 38 and a newor subsequent IOP is determined as represented by signal 39 formed byincreasing prism pressure from a preliminary value 27 b.

The prism pressure force used to elevate the IOP to test theaccommodating ability of the eye's trabecular meshwork is preferablysufficient to depress an ocular pulse signal during the predeterminedinterval that the prism force is applied. This results in theaccommodation attempted by the trabecular meshwork to arise from theelevated IOP caused by the prism force as distinct from periodicaccommodations following systolic pulses.

When elevated prism force is removed at the end of signal 37, the IOP ofthe previously pressurized eye reduces for a brief interval until theeye re-accommodates. During this interval a subsequent determination ofdiastolic IOP is made as represented by signal 39 showing that the IOPis reduced, and the eye has become temporarily softer. The differencebetween the higher IOP determined from signal 36 and the lower IOPdetermined from signal 39 gives a measure of the effectiveness of thetrabecular meshwork of the eye. The healthier the eye, the greater willbe the amount that the subsequent IOP is reduced from the preceding IOP.

This procedure is analogous to a known way of determining tonography bymeasuring IOP of an eye, and then while a patient is lying down, placinga weight on a patient's eye for an interval after which the IOP is againmeasured. Such a method is cumbersome and time consuming, since thepatient has to lie down, and a weight has to be placed on the eye andthen removed in between IOP determinations. By the method illustrated inFIG. 7, instrument 10 can make such a process much more efficientwithout requiring any weight and without requiring the patient to liedown.

1. A method of detecting possible constriction of blood flow in arteriessupplying a head, the method comprising: examining an eye of the headwith an applanation tonometer producing intra ocular pressure signalsrepresenting a diastolic phase signal and a departure from the diastolicphase signal occurring as a systolic pulse of blood enters the eye;determining blood flow to the eye from the systolic phase signal; anddetermining from the detected blood flow to the eye whether blood flowconstriction in arteries supplying the head may be occurring.
 2. Themethod of claim 1 including measuring blood flow to each of two eyes ofthe head to determine whether blood flow is significantly less in one ofthe eyes as an indication of where a possible blood flow constrictionmay be occurring.
 3. The method of claim 1 including subsequentlymeasuring blood flow in a carotid artery leading to an eye with asubnormal blood flow to determine whether blood flow is constricted inthe carotid artery.
 4. The method of claim 1 including programming thetonometer to distinguish between and to indicate normal and abnormalblood flow to the eye.
 5. A tonometer operated by the method of claim 1.6. A method of predicting possible cardiovascular events, the methodcomprising: examining an eye of a patient with an applanation tonometerto measure a systolic pulse of blood to the eye; determining an ocularpulse pressure (OPP) or ocular blood flow (OBF) from the systolic pulseto the eye; and determining whether either OPP or OBF is subnormal as anindication of possible constriction of a carotid artery leading to theeye.
 7. The method of claim 6 including programming the tonometer todistinguish between and to indicate normal and subnormal OPP and OBF. 8.The method of claim 6 including examining each eye of a patient with theapplanation tonometer, comparing OPP and OBF for each of the examinedeyes, and determining from the comparison whether one of the eyes has alower OPP or OBF than the other.
 9. The method of claim 6 includingexamining blood flow in a carotid artery leading to an eye having asubnormal OPP or OBF.
 10. A tonometer operated by the method of claim 6.11. The method of claim 10 wherein the tonometer includes amicroprocessor programmed to distinguish between and to indicate normaland subnormal OPP or OBF.
 12. A test for possible constriction of bloodflow in a carotid artery preliminary to a carotid artery ultrasoundexamination, the test comprising: examining each eye of a patient withan applanation tonometer that produces a systolic pulse signal as abolus of blood enters an eye being examined; determining ocular pulsepressure (OPP) or ocular blood flow (OBF) for each eye from the systolicpulse signals; and determining to proceed with a carotid arteryultrasound examination whenever the OPP or OBF determination for eithereye is subnormal.
 13. The test of claim 12 including determining not toproceed with a carotid artery ultrasound examination whenever the OPPand OBF are normal.
 14. A tonometer arranged to perform the test ofclaim 12.